No it is not a C. It is a Cat V1100, Serial Number: 6BF048.
It is a 1989 container lift truck with a spreader on it and air over hydraulic brakes. It is a 40,000 lb capacity to be able to move shipping containers ( very large industrial ones). The operator sits a story and a half off the ground.

At first, the brakes locked up. I rebuilt the brake pump with a new cartridge kit, and still did not have the recommended 400-500 psi. I did find that when the parking brake was engaged, the 500 psi's would return. But the parking brake is still locked up.

My question is is why? Has anyone had this experience?
I added 30 gallon of hydraulic oil to the tank so it is not that. I am guessing that when the parking brake is on, the pressure is bypassing somewhere... but where? Any ideas?

The dead man brake interlock under the seat on this particular type of lift (seat switch) was bypassed years ago by a different mechanic.

Someone suggested that maybe the brake seals are shot at the differential. Would that cause the brake to remain locked up? ( parking brake?)